go/printer, gofmt: don't align map entries for irregular inputs

Details: Until now, when we saw a key:value pair that fit onto
a single line, we assumed that it should be formatted with a
vtab after the ':' for alignment of its value. This leads to
odd behavior if there are more than one such pair on a line.
This CL changes the behavior such that alignment is only used
for the first pair on a line. This preserves existing behavior
(in the std lib we have composite literals where the last line
contains multiple entries and the first entry's value is aligned
with the values on previous lines), and resolves this issue.

No impact on formatting of std lib, go.tools, go.exp, go.net.

Fixes golang#8685.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/139430043

src/go/printer/nodes.go

@@ -163,8 +163,8 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 	size := 0
 
 	// print all list elements
+	prevLine := prev.Line
 	for i, x := range list {
-		prevLine := line
 		line = p.lineFor(x.Pos())
 
 		// determine if the next linebreak, if any, needs to use formfeed:
@@ -207,8 +207,8 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 			}
 		}
 
+		needsLinebreak := 0 < prevLine && prevLine < line
 		if i > 0 {
-			needsLinebreak := prevLine < line && prevLine > 0 && line > 0
 			// use position of expression following the comma as
 			// comma position for correct comment placement, but
 			// only if the expression is on the same line
@@ -232,16 +232,20 @@ func (p *printer) exprList(prev0 token.Pos, list []ast.Expr, depth int, mode exp
 			}
 		}
 
-		if isPair && size > 0 && len(list) > 1 {
-			// we have a key:value expression that fits onto one line and
-			// is in a list with more then one entry: use a column for the
-			// key such that consecutive entries can align if possible
+		if len(list) > 1 && isPair && size > 0 && needsLinebreak {
+			// we have a key:value expression that fits onto one line
+			// and it's not on the same line as the prior expression:
+			// use a column for the key such that consecutive entries
+			// can align if possible
+			// (needsLinebreak is set if we started a new line before)
 			p.expr(pair.Key)
 			p.print(pair.Colon, token.COLON, vtab)
 			p.expr(pair.Value)
 		} else {
 			p.expr0(x, depth)
 		}
+
+		prevLine = line
 	}
 
 	if mode&commaTerm != 0 && next.IsValid() && p.pos.Line < next.Line {

src/go/printer/testdata/declarations.golden

@@ -593,7 +593,7 @@ var (
 )
 
 func _() {
-	var privateKey2 = &Block{Type:	"RSA PRIVATE KEY",
+	var privateKey2 = &Block{Type: "RSA PRIVATE KEY",
 		Headers:	map[string]string{},
 		Bytes: []uint8{0x30, 0x82, 0x1, 0x3a, 0x2, 0x1, 0x0, 0x2,
 			0x41, 0x0, 0xb2, 0x99, 0xf, 0x49, 0xc4, 0x7d, 0xfa, 0x8c,
@@ -698,6 +698,29 @@ var _ = T4{
 	c:	z,
 }
 
+// no alignment of map composite entries if they are not the first entry on a line
+var _ = T{0: 0}	// not aligned
+var _ = T{0: 0,	// not aligned
+	1:	1,				// aligned
+	22:	22,				// aligned
+	333:	333, 1234: 12, 12345: 0,	// first on line aligned
+}
+
+// test cases form issue 8685
+// not aligned
+var _ = map[int]string{1: "spring", 2: "summer",
+	3:	"autumn", 4: "winter"}
+
+// not aligned
+var _ = map[string]string{"a": "spring", "b": "summer",
+	"c":	"autumn", "d": "winter"}
+
+// aligned
+var _ = map[string]string{"a": "spring",
+	"b":	"summer",
+	"c":	"autumn",
+	"d":	"winter"}
+
 func _() {
 	var _ = T{
 		a,	// must introduce trailing comma

src/go/printer/testdata/declarations.input

@@ -715,6 +715,31 @@ var _ = T4{
 }
 
 
+// no alignment of map composite entries if they are not the first entry on a line
+var _ = T{0: 0} // not aligned
+var _ = T{0: 0, // not aligned
+	1: 1, // aligned
+	22: 22, // aligned
+	333: 333, 1234: 12, 12345: 0, // first on line aligned
+}
+
+
+// test cases form issue 8685
+// not aligned
+var _ = map[int]string{1: "spring", 2: "summer",
+					3:             "autumn", 4: "winter"}
+
+// not aligned
+var _ = map[string]string{"a": "spring", "b": "summer",
+	"c": "autumn", "d": "winter"}
+
+// aligned
+var _ = map[string]string{"a": "spring",
+"b": "summer",
+	"c": "autumn",
+"d": "winter"}
+
+
 func _() {
 	var _ = T{
 		a,	// must introduce trailing comma